In the Pacific Northwest and British Columbia, southern resident orcas have experienced no net population growth since the 1970s, with just 73 left at the most recent count. But northern resident orcas, which have a similar diet, territory and social structure, have grown steadily, now numbering more than 300. A new study may help explain why: The two populations differ in how they hunt for salmon, their primary and preferred food source, a key difference that conservationists will have to take into account when designing interventions to help southern residents.